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PS:
as my thought,the first one is right,because there is only one subject case-- "puella".
and if translate to "because Diana loves the wreath" then I think the sentence will be "quia Diana coronam amat."

You are correct...note that Diana is in the accusative after quia, which means that the goddess is the direct object of the clause. Igitur, the girl loves Diana. Your translation of the second is also just right.